at headquarters. The third part was the Engineer Brigade, a volunteer organization, originally about 2,500 strong, under the command of a regular engineer, holding the rank of brigadier-general of volunteers.

During the past session of Congress a law had passed consolidating the Corps of Engineers and Topographical Engineers. No order, however, was published uniting the two offices at these headquarters till our return from Chancellorsville, so that my report is not designed to embrace the operations of the officer in charge of the Engineer Brigade, General Benham, nor those of Captain Comstock, of the Engineers.

The Engineer Brigade, under its previous commander, General Woodbury, made a great deal of accurate topographical reconnaissance along the Rappahannock River from Skinker's Neck to the Rapidan, particularly of that part above Fredericksburg and in the triangle of which that city, Hartwood Church, and the mouth of the Rapidan formed the vertices. A very considerable potion of the reconnaissance between Falmouth, Hartwood Church, Stafford Court-House, Aquia Landing, and King George Court-House was made under my predecessor in the Topographical Department, First Lieutenant Nicolas Bowen, Topographical Engineers (now Lieutenant-Colonel Bowen, assistant adjutant-general). Very extensive reconnaissances had been made in the campaign under General Pope by Captain W. H. Paine, aide-de-camp, and published under direction of Colonel Macomb, chief topographical engineer to General Pope, which extended from Manassas south to Rapidan Station.

The surveys about the defenses of Washington, under Major Whipple, topographical engineer (the late Major-General Whipple), form the northern limit of the map, and those on the operations on the Peninsula, by General Humphreys and Captain (now Colonel) Abbot, the southern limit. The coast surveys of the York and Rappahannock Rivers and reconnaissances of the Potomac River form the bases of the map.

I was placed in charge of the Topographical Engineers on February 2, 1863. All the other Topographical Engineers were then relieved, and Captain W. H. Paine and Lieutenant [Lebbeus H.] Mitchel were assigned to assist me, as well as several very competent non-commissioned officers and privates and a large force of civil assistants. Captain Paine especially aided me with much knowledge of the country, past experience, and zeal. With this force, every part of the country afterward occupied by our troops was mapped. Much information, too, was gained from negroes and deserters, and in a few cases by captures of maps from the enemy. All this is embodied in the map, to a scale of 1 mile per inch.

In order to make my report more easily understood and interesting, I will mention in brief some of the more general operations, of which mine were but a part, and give also a sketch of the situation. At the time the operations resulting in the battle of Chancellorsville and those attending it began, the enemy occupied in strong force the heights south of Rappahannock River, from Skinker's Neck to Banks' Ford, having continuous lines of infantry parapets throughout (a distance of about 20 miels), his troops being so disposed as to be readily concentrated on any threatened point. Interspersed along these lines of intrenchments were battery epaulements advantageously located for sweeping the hill slopes and bottom lands, on which our troops would have to march to the assault, and which effectually protected the enemy's artillery from our own. Abatis, formed of fallen timber, and impassable swamps in places, still further strengthened his lines and reduced the number of assailable points. The crests of the main hills, where the enemy had prepared to receive us, were from three-quarters to 1 1/2 miles